Insight on the global water distribution in opal considering its bi-component nature

Abstract

To date, studies on water distribution in opals (SiO₂.nH₂O, amorphous and porous) have considered opal exclusively in terms of silica structures (nanograins and aggregates such as spheres) without considering the, yet intrinsic, silica gel component. Consequently, its role in controlling both the water content and the distribution of water species (H₂O, SiOH) is still unresolved. In this study, Raman spectroscopy was applied to four calibrated synthetic opals representing varying ratios of silica structure and silica gel. The aim is to assess the nature of water in opal, especially regarding its bi-component nature. Our results show that an increase in the silica gel content in synthetic opals affects the content, type and proportion of water species by: (1) increasing the contribution of the bonded molecular water preferentially located in the porosity (H₂O type B) and the silanol groups present in the total amorphous structure (SiOH type A); (2) decreasing of the contribution of free molecular water (H₂O type A) and silanols groups adsorbed at the silica structure surface (SiOH type B). Moreover, the synthetic sample composed exclusively of silica structures (Op 1:0), which represent the theoretical model use to date, shows a systematic different behaviour to the other sample containing silica gel. All this exhibit that the silica gel phase plays an important role in the repartition of water in natural opals.